Yet another study links cesarean birth with asthma. This one involved 37,000 participants in the Norwegian Mother and Child Cohort Study, and compared cesarean- and vaginally-born children for evidence of asthma at age 3. Those born by cesarean section had an increased risk of asthma.

The authors speculate that the altered gut microbiota found in cesarean babies–the collection of bacteria that live in the bowel–may be the reason for the association. (See my posts here and here for an explanation of how and why an altered gut microbiota may be at the root of a number of later chronic illnesses.)

The study’s authors described the increased risk as “slight,” which contrasts with the “moderate” risk found by other researchers. This apparently lower risk may be due in part to the how the study was performed.

First, the researchers lumped all cesareans–both scheduled cesareans and those that followed a long labor, in which a baby may be exposed to the normal bacteria of the birth canal–rather than comparing scheduled cesareans to vaginal births. The latter comparison would give a clearer picture of childhood asthma risks from cesarean birth.

Second, the study only follows the children to 3 years of age. Many cases of asthma occur later in childhood, and a longer follow-up of these children (which is no doubt in the works) would give a clearer picture of the risks.

Studies like this one add more weight to the argument for reducing the number of cesareans currently being performed, particularly those done without any medical need. Women should be informed of the potential long-term health risks and benefits for their children when choosing how and where they want to have their babies.